Many optoelectronic devices such as LEDs are based on wide bandgap semiconductors, such as Group III-N semiconductors (e.g. GaN and its alloys with other Group III-N semiconductors). These devices have enabled new applications based on structures that use light with wavelengths in the blue to near-UV. The GaN semiconductor has an index of refraction n≅2.3. This is high compared to both air (n=1), and commonly used transparent insulators such as SiO2 (n=1.5) and Al2O3 (n=1.8). TiO2 is has an index of refraction which is too high at relevant wavelengths at about 3.2. ZnS is frequently used as a “high-index” optical material which is often quoted as having an index of refraction of 2.3 (at 632 nm), but it is unsuitable at lower wavelengths, both because the index of refraction is much higher, and because the absorption increases with decreasing wavelength. The net result is that there is often poor index matching at material boundaries for wide bandgap semiconductor optoelectronic devices, with associated reflective losses at the boundaries.
Silicon nitrides of varying stoichiometry are often used with GaN. Si3N4 has a nominal index of refraction of about 2.1 and by varying the stoichiometry, the index of refraction can be tuned over a range from about 1.8 to 2.5; however variations in the stoichiometry also affect the extinction coefficient such that typically absorption of light is increased for wavelengths in the ultraviolet and visible portions of the electromagnetic spectrum. There is a need for insulating materials which have indices of refraction close to that of GaN and also where the extinction coefficient for the material is negligible.
Flip chip blue LEDs have both n- and p-contacts on the same side of the chip, and therefore these two contacts need to be isolated from each other. An insulator commonly used today is silicon nitride (SiNx), usually deposited by plasma-enhanced chemical vapor deposition (PECVD). SiNx is used because it is a commonly deposited insulator, not because it is optically engineered for maximizing light output from the device. FIG. 1 shows a typical flip chip architecture. The metallic contacts are opaque and reflecting, and light is extracted from the device from the bottom of the device as oriented in FIG. 1. The p-contact area is typically the majority of the area of the device and is engineered to have high reflectivity. The region around the n-contact is not typically optimized for reflectivity as the p-contact due to the lack of materials available with optical properties that are optimized for this application.
One solution is to add a distributed Bragg reflector (DBR) to provide high efficiency light reflection in the vicinity of the n-contact. DBRs are complex structures requiring a plurality of precisely engineered layer pairs of alternating index of refraction having carefully controlled layer thicknesses. Carlin et al, describe the use of a DBR formed from pairs of layers of Al0.84In0.16N and GaN with near lattice matching between the layers (2003, Appl. Phys. Lett, 83, 668). Precise index matching to GaN is not required, but DBRs are expensive and difficult to make with high yields.
Bhat and Steigerwald (U.S. Pat. No. 6,630,689) describe a highly reflective dielectric stack by forming series of insulating layers with specified high and low indices of refraction to achieve a high reflectivity stack. However, the high reflectivity condition exists only for a limited range of incident angles at only at a specified wavelength of light.
There is a particular need for transparent insulators with a better index match to GaN to improve reduce reflective losses and improve light extraction from the vicinity of the n-contact, It is in this context that the insulators disclosed herein were developed, although it will be apparent to the skilled artisan that such index-matched materials can be used for other embodiments of optoelectronic devices.